1. Field of the Invention
The present invention relates to a liquid discharge apparatus having a liquid discharger capable of deflecting the trajectory of a droplet in various directions and relates to a method for discharging liquid. More specifically, it relates to a technology for shutting off the operation of a liquid discharger when the discharger fails to discharge droplets and for instructing another liquid discharger to discharge droplets as a replacement for of the shut-off discharger.
2. Description of the Related Art
An inkjet printer, which is a known liquid discharge apparatus, typically has a head including linearly-aligned liquid dischargers. By sequentially discharging minute ink droplets onto a recording medium, such as printing paper, disposed so that it opposes the nozzle surface, a predetermined number of dots are formed on the pixel area to form a pixel.
In some cases, the liquid discharger becomes incapable of discharging droplets normally. This is caused by various reasons.
One reason is discharge failure caused by dust blocking the droplet outlet of the nozzle of the discharger. A known method for solving this problem is head cleaning.
Another reason for discharge failure is caused by clogging of the liquid discharger or disconnection of an energizing element (for example, a heater element for a thermal printer) disposed inside the liquid discharger. In such cases, there are no sufficient methods for solving these problems. Thus, usually, the head is replaced.
There are two types of known inkjet printers: 1) a serial printer in which the head prints while it moves back and forth in the direction orthogonal to the feeding direction of the printing paper and while the printing paper is sent in the direction perpendicular to the direction in which the head is moving; and 2) a line printer in which a head is formed along the entire width of the printing paper and in which printing is performed while the printing paper is sent in the feeding direction.
A line inkjet printer has a plurality of small head chips disposed in parallel so that the ends of each head chip are connected. Each head chip is processed by an appropriate signal so that the head chips record data in series along the entire width of the printing paper.
A serial inkjet printer employs an overlapping method for printing a gray scale.
In this method, the characteristics of the liquid dischargers are averaged by overlapping ink droplets (dots) many times in one pixel area. More specifically, dots are formed to fill the gaps between the previously formed dot row.
Even if some of the liquid dischargers malfunction to a certain degree or if there are liquid dischargers that cannot discharge any liquid, the effect of failure of these liquid dischargers can be minimized by overlapping the dots.
The head of the line inkjet printer does not move in the direction perpendicular to the feeding direction of the printing paper. Therefore, once an area has been recorded, it cannot be re-recorded by overlapping the dots.
When there is more than one failed discharger, a white line is generated because a row of pixels cannot be formed. This failure is prominent in photographs and graphics that require high quality.
For line inkjet printers, the tone can be increased by overlapping dots in the feeding direction of the printing paper. Overlapping of the dots, however, is only effective for increasing the tone and does not average the quality of the dischargers.